docs: def set_optional_elements(self):
          """Sets elements considered option by RSS spec"""
          self.set_categories()
          self.set_copyright()
          self.set_generator()
          self.set_image()
          self.set_language()
          self.set_last_build_date()
          self.set_managing_editor()
          self.set_published_date()
          self.set_pubsubhubbub()
          self.set_ttl()
          self.set_web_master()